The subscription television system is well known in the art in which only the contractor can watch a specified broadcasting program on TV. That is, in the subscription television system, the specified broadcasting program is sent out on the transmitting side in the form of the scrambled TV signal in which the synchronizing pulse interval is suppressed a certain level. In case this scrambled TV signal is received by the usual TV set, the TV set can not be synchronized because the synchronizing pulse is suppressed, and the normal image can not be watched. On the contrary, the contractor can receive the normal picture by employment of a descrambler. The descrambler performs its descrambling process, by which the video signal interval of the received scrambled TV signal, except for the synchronizing pulse interval, is suppressed a certain level. This descrambled TV signal is applied to the TV set.
In more detail, FIG. 4(a) shows an example of a high-frequency envelop wave form of the scrambled TV signal. In this wave form the interval A inclusive of the horizontal synchronizing pulse HP and the color burst CB is suppressed a certain level. In addition, the interval of the vertical synchronizing pulse is also suppressed a certain level. The sound carrier is amplitude-modulated by a timing pulse signal shown in FIG. 4(b) and sent out together with a video signal.
On the receiving side, the timing pulse signal shown in FIG. 4(b) is extracted to create a control pulse signal shown in FIG. 4(c), the descrambling process is performed so that the video signal interval, except for the synchronizing pulse interval, is suppressed a certain level to create the TV signal having the normal relationship of levels, and the resultant TV signal is applied to the TV set.
FIG. 5 is a block diagram of the important portion of the conventional descrambler. In this drawing, the scrambled TV signal applied to an input terminal 8 is converted to a certain frequency by a frequency converter 1, and branched by a branching circuit 2 into two, one being applied to an amplifier 3 and the other being applied to a detector circuit 7. The timing pulse signal obtained through detection in the detector circuit 7 is applied to a decoder section 6, thereby the control pulse signal shown in FIG. 4(c) being created, which in turn is applied to switching circuits 10, 11. In operation, the switching circuits 10, 11 are turned to the contact a side during the synchronizing pulse interval, or these are turned to the contact b side during the video signal interval.
Attenuators 4, 5 are previously set to mutually different values of attenuation, the decoder section 6 controls switching circuits 12, 13 in response to the predetermined control information given from the transmitting side, and the switching circuits 12, 13 are turned and connected to either contact c or d so that either attenuator having a designated value of attenuation is connected to the switching circuits 10, 11. Accordingly, the output signal of the amplifier 3 during the synchronizing pulse interval is given to and output through an output terminal 9 without modification, whereas the output signal of the amplifier 3 during the video signal interval is attenuated a certain level by the attenuator 4 or 5 and output through the output terminal 9, whereby the normal level relationship is recovered between the synchronizing pulse interval and the video signal interval.
As apparent from the foregoing description, in the prior art, the attenuation values of the attenuators 4, 5 are required to be set in correspondence with the value of suppression on the transmitting side and, if the set attenuation values vary, the quality of received picture deteriorates. Specifically, against the scramble TV signal given from the transmitting side upon which the descrambling process is to be performed by connecting selectively plural attenuators 4, 5, the attenuation values of the attenuators 4, 5 must be set accurately. If the set attenuation values vary due to secular change or variation in temperature, a defect such as deterioration of picture quality or flashing of picture results.